How to make objects in foreground appear darker than the background?
There are two fast ways this could be accomplished. 

The first way is through lighting. Make the background brighter, and don't cast nearly as much light on the foreground. The light would still bounce around, lighting up the foreground a tad bit, but that's natural and would still look good.
To emphasize this effect even more, turn up the brightness of the background lights, and turn down the exposure.

The second way would be to use render layers, and composite the foreground over the background using an alpha over node. This way, you could finely tweak the colors, saturation, and value of both the foreground and background individually, making awesome results. So, basically, you'd darken the foreground to almost nothing, but keep the value of the background and maybe even boost saturation.

Hope this gets you going in the right direction!